Background <p>Hydronephrosis is a common finding on renal ultrasound, but its physiological causes-such as hydration-induced distention-remain under-recognized in healthy individuals. Misinterpreting such cases as pathological may lead to unnecessary investigations. This study aimed to determine whether rapid oral hydration induces transient hydronephrosis in healthy adults, and to characterize its timing, laterality, and reversibility.</p> Methods <p>Fifty-six healthy volunteers (30 males, 26 females; aged 19–45 years) without chronic illnesses underwent baseline renal and bladder ultrasound, followed by rapid oral hydration (20&#xa0;ml/Kg within 15–30&#xa0;min). Ultrasound scans were repeated at 60 and 90&#xa0;min post-hydration, before and after voiding. Hydronephrosis was graded using the Radiology classification system. Statistical significance was defined as <i>p</i> &lt; 0.005.</p> Results <p>Fifty-six healthy adults (30 males, 26 females; aged 19–45 years) were enrolled in the study. At 60&#xa0;min, 46.6% of participants developed hydronephrosis, increasing slightly to 48.2% at 90&#xa0;min. A significant increase in hydronephrosis grade over time was observed (<i>p</i> = 0.0005), predominantly affecting the right kidney (<i>p</i> &lt; 0.001). Post-void imaging showed partial resolution (<i>p</i> = 0.0003), but moderate hydronephrosis persisted in 12.5% at 90&#xa0;min. Bladder volume did not significantly correlate with hydronephrosis severity. Although males had larger kidneys and bladder volumes, there was no significant gender difference in hydronephrosis incidence.</p> Conclusions <p>Rapid hydration can induce transient, non-obstructive hydronephrosis in healthy adults, especially on the right side. Radiologists and clinicians must recognize this physiological variant to avoid misdiagnosis and unnecessary work-up in asymptomatic patients undergoing renal ultrasound.</p> Clinical trial number <p>Not applicable.</p>